This invention generally is directed to a method of treating flue gas and other industrial waste gases to efficiently and economically neutralize or remove therefrom a substantial portion of the acid-forming gaseous component unfortunately resulting from combustion of fuels and processing organic waste materials. The invention is especially directed to a process for treating gaseous mixtures which contain oxides of sulfur and nitrogen, hereinafter referred to as SO.sub.x and NO.sub.x.
Worldwide energy supply problems unfortunately have mandated burning larger quantities of fuels having higher sulfur content or chlorinated waste materials and the like. The acidic components of the resulting flue gases now present an extremely serious environmental hazard. Aside from the more readily apparent problems resulting from corrosion and contamination of the air which must be breathed, the acidic components of flue gas are considered as the major source of the "acid rain" currently threatening the ability of many lakes in North America to support life. It is not surprising therefore, that industry has engaged in an intensive search for efficient and economical methods of neutralizing the noxious acidic components of waste flue gases.
SO.sub.x and NO.sub.x may be contained in practically all flue gases. When coal, oil or gas is burned, the sulfur contained in such fuels is converted to SO.sub.x and becomes part of the flue gases. The NO.sub.x found in flue gases comes from two sources: either from the thermofixation of atmospheric nitrogen during the combustion process or from the chemically bound nitrogen in the fuel. In natural gas firing nearly all NO.sub.x results from thermofixation. In combustion of residual oil, crude oil, and coal, the contribution from fuel-bound nitrogen can be significant and with some fuels, predominant.
For the most part, SO.sub.x and NO.sub.x emission control have been handled as separate entities. Although efforts to effect simultaneous removal of these materials are reflected in the literature there is today no really satisfactory process which will efficiently and economically remove SO.sub.x and NO.sub.x from waste gases without producing water soluble reaction products and which thereafter will permit safe discharge of the "clean" flue gas to the atmosphere. For example, in U.S. Pat. No. 3,880,618 to McCrea et al. a regenerative process is disclosed wherein flue gas first is passed over alkalized alumina or an alkali metal oxide or carbonate at 75.degree.-150.degree. C. to simultaneously remove SO.sub.x and NO.sub.x. The absorbant then is heated to 300.degree.-400.degree. C. to drive off NO, then heated to 600.degree.-700.degree. C. to drive off absorbed sulfur compounds. The absorbant then is cooled and recycled to the absorption step. No suggestion is made that an alkaline earth reagent such as calcium oxide of calcium hydroxide would be useful in the process. On the other hand, in Kyllonen U.S. Pat. No. 3,498,743, a process is taught for removal of NO.sub.2 from "other gases" by contacting the gaseous mixture with alkali or alkaline earth metal carbonates or oxides in a fixed or fluid bed between ambient temperature and 200.degree. C. in the presence of between 1000 and 8000 p.p.m. water vapor. However, Kyllonen nowhere suggests that NO.sub.x and SO.sub.x can be captured or removed in his process.
Prior art flue gas treating processes and especially desulfurization processes, have utilized a variety of absorbants including ammonia and the oxides, hydroxides and carbonates of the alkali and alkaline earth metals. Various absorbant flue gas contacting means have been used including solution or slurry/gas contacting absorption towers and various spray/gas contacting systems. Prior art processes often have been differentiated by the nature of the absorbant/SO.sub.x reaction product effluent. Thus, a "wet" sludge is produced from absorption scubbing towers and a "dry" reaction product is produced from spray drier absorption processes.
Flue gas NO.sub.x control means normally operate either through suppression of NO.sub.x formation or through physical or chemical removal of NO.sub.x from the flue gases. NO.sub.x formation can be suppressed by reducing the nitrogen level at peak flame temperature; decreasing oxygen level at peak flame temperature or by reducing peak temperature and residence time in the combusion zone. Chemical removal means have included both dry and wet processes. Dry processes include (1) catalytic decomposition, (2) selective catalytic reduction of NO.sub.x with NH.sub.3, (3) non-selective catalytic reduction with reducing gases, (4) non-catalytic reduction NH.sub.3 and (5) absorption by solids as in U.S. Pat. No. 3,498,743. The wet processes include (1) absorption in a liquid phase and oxidation of NO.sub.x to NO.sub.2 /NO.sub.3, (2) gas phase oxidation followed by absorption and liquid phase reduction, (3) gas phase oxidation followed by absorption and liquid phase oxidation to NO.sub.2 /NO.sub.3 and (4) absorption with a liquid phase reduction to NH.sub.4+ .
Both dry and wet processes for the simultaneous removal of SO.sub.x and NO.sub.x are described in the literature. The dry processes include: (1) selective catalytic reduction with ammonia and absorption of SO.sub.x by activated carbon; (2) selective catalytic reduction with ammonia and reaction of SO.sub.2 with copper oxide; (3) absorption of NO.sub.x and SO.sub.x by solids such as alkalized alumina, sodium or potassium oxide or carbonate or ores thereof as in U.S. Pat. No. 3,880,618; and (4) electron beam radiation. The wet processes include: (1) absorption of NO.sub.x and SO.sub.2 with liquid phase reduction of NO.sub.x to N.sub.2 by SO.sub.2 ; (2) absorption of NO.sub.x and SO.sub.2 with liquid phase oxidation to NO--.sub.3 and SO.dbd..sub.4 ; (3) gas phase oxidation of NO.sub.x and SO.sub.x with liquid phase reduction of NO.sub.x to N.sub.2 ; and (4) gas phase oxidation of NO.sub.x and SO.sub. x with liquid phase oxidation of NO.sub.x to NO--.sub.3. These processes are reviewed in an article by A. A. Siddiqi and J. W. Tenni of Arco Petroleum Products Company published in Hydrocarbon Processing, October, 1981. In this article, most flue gas treatment technology currently under commercial development is reviewed, providing good background material.
Most development of SO.sub.x /NO.sub.x simultaneous removal has been conducted with the dry processes, especially selective reduction with NH.sub.3, due to the complexity and cost of the wet processes. Wet NO.sub.x removal processes generally are not now considered economically viable as compared to the dry processes.
In general, dry flue gas treatment processes have the following advantage over wet processes:
I. Lower capital investment and lower annual revenue requirements; PA1 2. Simpler processes and fewer equipment requirements; PA1 3. Higher NO.sub.x removal efficiency; PA1 4. More extensive testing in large units (mainly oil and gas fire boilers); PA1 5. No waste generation. PA1 I. Greater sensitivity to inlet particulate matter levels; PA1 2. Requirements for NH.sub.3 from either an energy source (natural gas) or more expensive coal gasification methods; PA1 3. Possible emission of (NH.sub.4).sub.2 SO.sub.4, NH.sub.4 HSO.sub.4 and NH.sub.3 ; and PA1 4. Higher reaction temperature (350.degree.-400.degree. C.) which requires these systems to be placed in the power generation cycle before the boiler preheater or ancillary heating after the preheater. PA1 I. Simultaneous SO.sub.x /NO.sub.x removal, which may provide economic advantage under certain applications; PA1 2. Relative insensitivity to flue gas particulate matter; PA1 3. High SO.sub.x reduction. PA1 I. Higher costs because of insolubility of NO in aqueous solutions; PA1 2. Formation of nitrates and other potential water pollutants; PA1 3. More extensive equipment requirements; PA1 4. Formation of low-demand by-products; PA1 5. Need for flue gas preheat; PA1 6. Moderate NO.sub.x removal efficiencies; PA1 7. Usually are limited to high SO.sub.x /NO.sub.x ratios. PA1 I. The specific reagent created to capture both SO.sub.x and NO.sub.x simultaneously; PA1 2. The highly specific reaction conditions found necessary to accomplish the simultaneous capture of SO.sub.x and NO.sub.x ; PA1 3. The relatively attractive nature of the solid by-products formed; PA1 4. The relative cost of process capital requirements; and PA1 5. The relative simplicity and reliability of the operation.
However, dry processes available to date also have the following disadvantages:
In general wet SO.sub.x /NO.sub.x processes currently available have the following advantages as compared to dry processing:
Major disadvantages of the wet processes include: